Typically, an optical storage element is defined as those which use optical and electrical means to read or write signals stored or to be stored therein. Currently available optical storage elements include audio cassette recorders (and tapes), video cassette recorders (and tapes), albums, optical discs, cameras, films, etc. The audio recording medium has been improved from the conventional plastic albums to magnetic tapes and compact disks.
Essentially all the commercially available compact disks (CD) or laser discs (LD) are read-only or write-only. Once the data is stored therein, it is impossible to do compilation, erasal, or modification thereof. This is a very inconvenient and uneconomical way to store data. However, because of the fundamental shortcomings of the current optical recording medium, unless new recording media are developed, the situation will continue.
Currently, research efforts in the optical recording industry are focusing on the development of new materials to optically store information therein. For instance, V. P. Shibaev, et al. disclosed a liquid crystalline polymer in "Thermo Recording on Liquid-Crystalline Polymers with Aid of a Laser Beam", Polym. Commun., 1983, Vol. 24, December, pp. 364-365., as well as in a serial of research papers published subsequently. Dubois, J.-C., G. Decobert, P. Le Barny Thomson, and D. de Corbeville disclosed liquid crystalline side chain polymers in "Liquid Crystalline Side Chain Polymers Derived from Polyacrylate, Polymethyacrylate and Poly-.alpha.-Chloroacrylat", Mol. Cryst. Liq. Cryst., 1986, Vol. 137, pp. 349-364. All those research publications show that some crystal liquids can exhibit various liquid crystal phases through the external application of an electric field. Most importantly, they demonstrated that these changes are reversible. These discoveries, however, still require the development of special liquid crystal material in order to provide optical memory capability for use in storing data.
Conventionally, liquid crystal polymers are synthesized based on liquid crystal monomers having chiral benzene ring or chiral biphenyl ring, graft-polymerized with optically active polymethylsiloxane compound to form side-chain liquid crystalline polymer having a molecular weight less than about 10,000. The side-chain liquid crystalline polymers often exhibit the properties of spontaneous polarization (PS), electro-optical responsiveness and optically bistable states.
A liquid crystal composition can be used in an optical data storage device because of the electro-optical responsiveness property thereof. The molecular arrangement in a liquid crystal compound generally can be classified into two types--the nematic type and the smectic type. Different types of liquid crystal compounds exhibit different electro-optical effect. The nematic type of liquid crystal has been widely used in optical-electrical storage device. The smectic type of liquid crystal has been used in a ferroelectric liquid crystal devices (FLCD). The latter type of liquid crystal has not been widely used because of the lack of technology to control the thickness of the liquid crystal material as well as the stability of alignment, etc. thereof. The smectic type of liquid crystal has been reported to exhibit excellent optical data storage capabilities in several publications, e.g., Finkelmann, H., D. Naegele and H. Ringsdorf, "Orientation of Nematic Liquid Crystalline Polymers in the Electric Field", Makromol. Chem. 180, 1979, pp. 803-806; Kaempf, G., B. Ag, H. Loewer and M. W. Witman, "Polymers as Substrates and Media for Data Storage", Polym. Eng. and Sci., October, 1987, Vol. 27, No. 19, pp. 1421-1435; McArdle, C. B., M. G. Clark, C. M. Haws, M. C. K. Wiltshire, A. Parker, G. Nestor, G. W. Gray, D. Lacey and K. J. Toyne, "Laser Addressed Thermo-Optic Effect in a Novel Dyed Liquid-Crystalline Polysiloxane", Liquid Crystal, 1987, Vol. 2, No. 5, pp. 573-584. However they only can be used in electro-optical devices since their structural optical scattering center will be shifted when the external electrical field or the surrounding temperature, etc. changes. This causes the stored data to be damaged during the realigned of the molecular structure. Most of application cannot allow the presence of such unstability. That is a principle reason why there has not been a practical application yet.
In 1988, Koide, N., K. Uehara and K. limura published an article entitled: "Synthesis and Thermal Properties of Side-Chain Type Liquid Crystalline Polymers with Chiral Unit in the Flexible Moiety", Mol. Cryst. Liq. Cryst., 1988, Vol. 157, pp. 151-162. In that article it is disclosed that the storage stability in ferroelectric liquid crystal can be enhanced by applying a laser beam on such a material to store data therein, then converting its molecular alignment into a special structure by "freezing the material" at a temperature above the glass transition temperature of the main chain thereof. However this article never teaches or suggests whether or how it is possible to erase or modify the data store therein.